Laurite and zircon from the Finero chromitites (Italy): New insights into evolution of the subcontinental mantle |
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| Institution: | 1. Department of Geochemistry and Ore-Forming Processes, A.N. Zavaritsky Institute of Geology and Geochemistry, Ural Branch of the Russian Academy of Sciences, Vonsovsky str. 15, Ekaterinburg 620016, Russia;2. Australian Research Council Centre of Excellence for Core to Crust Fluid Systems (CCFS)/GEMOC Key Centre, Department of Earth and Planetary Sciences, Macquarie University, Sydney, NSW 2109, Australia;3. NATI Research JSC, Otechestvennaya 3-3a, St. Petersburg 195030, Russia;1. State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing 100083, China;2. School of Earth Science and Resources, China University of Geosciences, Xueyuan Road, Haidian District, Beijing 100083, China;3. Department of Earth Sciences, University of Adelaide, Adelaide, SA 5005, Australia;1. Faculty of Land and Resource Engineering, Kunming University of Science and Technology, Kunming 650093, China;2. CSIRO Mineral Resources, Clayton, Vic 3168, Australia;3. Xi’an Institute of Geological and Mineral Exploration, Xi’an 700100, China;4. Sinopec Exploration Company, Chengdu 610041, China;1. Institute of Geomechanics, Chinese Academy of Geological Sciences, Beijing 100081, China;2. State Key Lab of Geological Processes and Mineral Resources, China University of Geosciences (Beijing), Beijing 100083, China;3. School of Earth Sciences and Resources, China University of Geosciences (Beijing), Beijing 100083, China;4. Institute of Resources and Environment, Hebei Geo University, Hebei 050031, China;1. Sciences de la Terre, Université du Québec à Chicoutimi, QC G7H 2B1, Canada;2. Department of Earth and Planetary Sciences, McGill University, QC H3A OE8, Canada;3. United States Geological Survey, Spokane Office, WA 99201, USA;1. Institut für Geowissenschaften, Goethe-Universität, Frankfurt am Main, Germany;2. Department of Geology, University of Johannesburg, PO Box 524, Auckland Park, 2006 Johannesburg, South Africa |
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| Abstract: | Chromitites enclosed within metasomatised Finero phlogopite peridotite (FPP) contain accessory platinum-group minerals, base metal (BM) sulfides, baddeleyite, zircon, zirconolite, uraninite and thorianite. To provide new insights into mantle-crustal interaction in the Finero lithosphere this study evaluates (1) the mineral chemistry and Os-isotope composition of laurite, (2) the crystal morphology, internal structure, in-situ U-Pb, trace-element and Hf-isotope data of zircon from two chromitite localities at Alpe Polunia and Rio Creves. The osmium isotope results reveal a resticted range of ‘unradiogenic’ 187Os/188Os values for laurite at Alpe Polunia (0.1247–0.1251, mean 0.1249 ± 0.0001). Re-Os model ages (TRD) of laurite reflect an Early Paleozoic partial melting event (ca 450 Ma or older), presumably before the Variscan orogeny. The Os isotopic composition of laurite/chromitite probably preserves their mantle signature and was not affected by later metasomatic processes. U-Pb and Hf-isotope data show that the Finero chromitites have distinct zircon populations with peculiar morphology, internal cathodoluminescence textures, trace-element composition and an overall U-Pb age span from ～310 Ma to 190 Ma. Three age peaks at Rio Creves (220 ± 4 Ma, 234.2 ± 4.5 Ma and 277.5 ± 3.2 Ma) are consistent with a prolonged formation and multistage zircon growth, in contrast to the common assumption of a single metasomatic event during chromitite formation. The trace-element signatures of zircons are comparable with those of mantle-derived zircons from alkaline ultramafic rocks, supporting the carbonatitic nature of the metasomatism. Hf-isotope compositions of the Finero zircons, with εHf values ranging mainly from ?3 to +1, are consistent with crustal input during metasomatism and could indicate that the parental melts/fluids were derived from a relatively old source; the minimum estimates for Hf model ages are 0.8–1.0 Ga. Our findings imply that mantle rocks and metasomatic events at Finero have a far more complex geological history than is commonly assumed. |
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| Keywords: | Laurite Zircon Chromitite Age Hf-Os-isotopes Subcontinental mantle Metasomatism Finero |
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